Boiler water can vary widely in its mineral content and, therefore, vary over a wide range of resistivities. Water which is relatively free of mineral deposits has a relatively high resistance and is more desirable in boiler applications as there is less boiler scale generated or deposited by the use of the pure form of water. In many boilers, the water contains a substantial amount of minerals due to the local water conditions, and this type of water has a tendency to build up a scale across the probe element thereby simulating the presence of water. Normal boiler water probes may require regular maintenance to clean the boiler scale from the probe or require a probe design that physically tends to expand and contract to tend to be self-cleaning. The use of a boiler water probe using a single resistive circuit for measuring the presence or absence of boiler water is relatively inexpensive, but is also less reliable than would be desirable in most boiler installations.
Boiler water sensing mechanisms also have been developed which use floats to sense the level of boiler water and mechanically operate switches. This type of mechanism also is subject to corrosion and boiler water scale contamination in the mechanism of the float, thereby creating a sensing mechanism which also may be less than desirable in reliability.